A photovoltaic (PV) process basically consists of four steps: (1) absorption of a photon, (2) charge separation, (3) charge transport and (4) charge collection. Thin-film technologies with low cost potential are compared with crystalline silicon in the table below, which also includes the compounds used for each step of the above-mentioned photovoltaic process.
Charge separationCharge transportCharge collectionLight(n/p-P = p-type SC*C = cathodeabsorberjunction)N = n-type SC*A = anodeCrystallineSiSi:P(n-type)/P = p-type SiC: AlSip-type SiN = Si:PA: Metal gridThin filmSiSi:P(n-type)/P = p-type SiC: AlSip-type SiN = Si:PA: ITO, ZnO: Al, SnO2: FAmorphousSiSi:P(n-type)/P = p-type SiC: AlSiintrinsic Si/N = Si:PA: ITO, ZnO: Al, SnO2: Fp-type SiCu(In, Ga) -Cu(In,Ga)CuIn(Se, S)2 orN: CuIn(Se, S)2C: Mo(Se, S)2(Se, S)2CdS/CuGa(Se, S)2or CdSA: ITO, ZnO: Al, SnO2: FP: CuGa(Se, S)2CdTeCdSCdS/CdTeN: CdSC: Al, Cu(& CdTe)P: CdTeA: ITO, ZnO: Al, SnO2: FDyeRu-dye,TiO2/dyeN: TiO2C: Al, ITO, ZnO: Al,sensitizedOrganicP: I−I3−, CuI,SnO2: FdyeSpiro-OMeTADA: ITO, ZnO: Al, SnO2: FOrganic(MEH)PPVC60/(MEH)PPVN: C60C: AlP: (MEH)PPVA: ITO*SC = semiconductor
Although all the above mentioned concepts are categorized as thin film photovoltaic devices, only the pure organic technology uses really thin films for the photoactive layer (<200 nm). The other technologies use film thicknesses of the photoactive layer between 1 and 20-30 μm. Such thick films require high temperature manufacturing steps to realize sufficient charge mobilities in the respective n-and p-type semiconducting charge transporting materials. Otherwise, no charges could be collected at the electrodes. Although charge mobilities are low in organic photovoltaic technology, charges can be successfully collected due to the very thin photoactive layer (<200 nm).
Although energy conversion efficiencies of 2.5% and 2.9% were reported in 2001 by Shaheen et al. in Applied Physics Letters, volume 78, pages 841-843; and by Munters et al. in the Proceedings of E-MRS Spring meeting, the main drawback for organic photovoltaic devices remains the stability of the device.
Since the beginning of the 80's, much research effort has been devoted to so-called quantum dots or inorganic nano-particles. Several photovoltaic devices had been reported before Huyn et al. disclosed a energy conversion efficiency of 1.7% in Science, volume 295, pages 2425-2427, using a blend of nano-rods of CdSe (as light absorber and n-type semiconducting material) and poly-3(hexylthiophene) (as light absorber and p-type semiconducting material). Although the photovoltaic blend can be applied in a single coating step, resulting in a layer thickness of 110 nm, the blend still contains an organic semiconducting material, and hence stability could still be a problem.
The so-called dye sensitized solar cell reported in 1991 by Graetzel in Nature, volume 353, pages 737-740, and disclosed in U.S. Pat. No. 4,927,721, U.S. Pat. No. 5,350,644 and JP-A 05-504023, is also a type of bulk heterojunction photovoltaic cell in which TiO2 nano-particles are used as the n-type semi-conductor. However, the construction of these devices is far more complicated than a one-step coating of a photovoltaic layer.
A need therefore exists for novel thin film photovoltaic materials.
GB 1119372 discloses a photovoltaic device based on Cu2S-powder which is pressed at 700-1000° C. and 100-1000 kg/cm2 to form a plate 2.0 cm2 in area and 3.0 mm thick. A CdS plate is prepared by pressing CdS powder into pellets, sintering at 800° C. in a nitrogen atmosphere, and grinding to a powder. The product is then pressed as above to form a plate 0.75 cm2 in area and 0.35 mm thick. After etching and polishing the surfaces, both disks were placed in an alloy die, enveloped in powder graphite, and pressed at 400° C. and 200 kg/cm2. This photovoltaic device can be described as a two-dimensional p-n heterojunction device. ZnSe or ZnS could be also used instead of CdS, but with reduced light absorption due to the larger bandgaps.
In 2002 Reijnen et al.in Biomimetic and Supramolecular Systems, volume C19(1-2), pages 311-314, reported the feasibility of a photovoltaic device with TiO2 as n-type semiconductor and vacuum deposited Cu1.8S as a p-type semiconductor and absorber.
In 1968, Vlasko et al. reported in Phys. Stat. Sol., volume 26, pages K77-80, investigated the possible role of p-n heterojunction formation in CuxS and ZnS evaporated layer structures exhibiting electroluminescence.
Coprecipitation of copper sulphide and zinc sulphide was reported in 1932 by Kolthoff et al. in Journal of Physical Chemistry, volume 36, pages 549-566. In 1998, Tsamouras et al. in Langmuir, volume 14, pages 5298-5304, described the preparation and characterization of mixed Cu(II), Zn(II) sulphides which they described by the stoichiometric formula CuxZn1-xS obtained by spontaneous precipitation in electrolyte solutions upon mixing copper and zinc nitrate solutions with an ammonium sulphide solution at a pH of 2.5 with small quantities of hydrazine sulphate being added to accelerate sulphide precipitation. XRD-spectra showed reflections of both ZnS and CuS, the precipitates mainly consisting of the two sulphides together with nonstoichiometric sulphides of Cu(II) and Zn(II). The spherullitic particles obtained upon increasing zinc content had a mean particle size of 0.5 to 1 μm. The precipitated copper(II) sulphide exhibited metallic conductor behaviour over the temperature range investigated. The precipitates behaved as n-type semiconductors. No photovoltaic evaluations were reported.
In 1999, Tsamouras et al. in Langmuir, volume 15, pages 7940-7946, reported the properties of a series of CuxNi1-xS powders prepared by spontaneous coprecipitation in aqueous solutions by rapidly mixing copper(II) and nickel(II) chloride solutions with a ammonium sulphide solution at a pH of 2.5 with hydrazine sulphate being finally added at a concentration equal to the final sulphide concentration in the solution to accelerate sulphide formation. Powder XRD spectra included new peaks which could not be attributed to copper(II) sulphide, nickel(II) sulphide or other known species suggesting the formation of new intermediate phases. The powders exhibited broad particle size distributions with mean particle sizes of 5 to 11 μm. Current potential curves in electrochemical cells with CuxNii-xS/Au and Pt electrodes with 0.01 M Ce3+/Ce4+ as electrolyte under constant tungsten-halide lamp illumination (100 mW/cm2) gave values for the open circuit potential as high as 0.868 V with 0.119 mA for the photocurrent and 0.58 for the field factor.
EP-A 1 231 250 discloses a method for manufacturing a thin film inorganic light emitting diode device, said method comprising following steps: (1) preparing a nano-particle dispersion comprising together ZnS doped with a luminescence centre (n-type semiconductor) and CuxS (p-type semiconductor) by precipitation from appropriate aqueous solutions of the respective ions, or, (1′) preparing a first separate nano-particle dispersion of ZnS doped with a luminescent centre (n-type semiconductor) and a second separate nano-particle dispersion of CuxS (p-type semiconductor), both by precipitation from appropriate aqueous solutions of the respective ions, (2) washing the dispersion prepared according to (1) or both dispersions according to (1′) to remove non-precipitated ions, (3) coating onto a first conductive electrode the dispersion resulting from steps (1) and (2), or a mixture of the dispersions resulting from steps (1′) and (2) in one and the same layer, or the separate dispersions resulting from steps (1′) and (20 in two separate layers, (4) applying on top of said coated layer(s) resulting from step (3) a second conductive electrode, with the proviso that at least one of said first and second electrodes is transparent.